TypeTags

A TypeTag[T] encapsulates the runtime type representation of some type T. Like scala.reflect.Manifest, the prime use case of TypeTags is to give access to erased types. However, TypeTags should be considered to be a richer replacement of the pre-2.10 notion of a Manifest, that are, in addition, fully integrated with Scala reflection.

There exist three different types of TypeTags:

scala.reflect.api.TypeTags#TypeTag.
A full type descriptor of a Scala type. For example, a TypeTag[List[String]] contains all type information, in this case, of type scala.List[String].
scala.reflect.ClassTag.
A partial type descriptor of a Scala type. For example, a ClassTag[List[String]] contains only the erased class type information, in this case, of type scala.collection.immutable.List. ClassTags provide access only to the runtime class of a type. Analogous to scala.reflect.ClassManifest
scala.reflect.api.TypeTags#WeakTypeTag.
A type descriptor for abstract types (see description below).

Like Manifests, TypeTags are always generated by the compiler, and can be obtained in three ways:

#1 Via the methods typeTag, classTag, or weakTypeTag

For example:

import scala.reflect.runtime.universe._
val tt = typeTag[Int]

import scala.reflect._
val ct = classTag[String]

Each of these methods constructs a TypeTag[T] or ClassTag[T] for the given type argument T.

#2 Using an implicit parameter of type `TypeTag[T]`, `ClassTag[T]`, or `WeakTypeTag[T]`

For example:

import scala.reflect.runtime.universe._

def paramInfo[T](x: T)(implicit tag: TypeTag[T]): Unit = {
  val targs = tag.tpe match { case TypeRef(_, _, args) => args }
  println(s"type of $x has type arguments $targs")
}

scala> paramInfo(42)
type of 42 has type arguments List()

scala> paramInfo(List(1, 2))
type of List(1, 2) has type arguments List(Int)

#3 Context bound of a type parameter

...on methods or classes. The above example can be implemented as follows:

import scala.reflect.runtime.universe._

def paramInfo[T: TypeTag](x: T): Unit = {
  val targs = typeOf[T] match { case TypeRef(_, _, args) => args }
  println(s"type of $x has type arguments $targs")
}

scala> paramInfo(42)
type of 42 has type arguments List()

scala> paramInfo(List(1, 2))
type of List(1, 2) has type arguments List(Int)

`WeakTypeTag`s

WeakTypeTag[T] generalizes TypeTag[T]. Unlike a regular TypeTag, components of its type representation can be references to type parameters or abstract types. However, WeakTypeTag[T] tries to be as concrete as possible, i.e. if type tags are available for the referenced type arguments or abstract types, they are used to embed the concrete types into the WeakTypeTag[T].

Continuing the example above:

def weakParamInfo[T](x: T)(implicit tag: WeakTypeTag[T]): Unit = {
  val targs = tag.tpe match { case TypeRef(_, _, args) => args }
  println(s"type of $x has type arguments $targs")
}

scala> def foo[T] = weakParamInfo(List[T]())
foo: [T]=> Unit

scala> foo[Int]
type of List() has type arguments List(T)

TypeTags and Manifests

TypeTags correspond loosely to the pre-2.10 notion of scala.reflect.Manifests. While scala.reflect.ClassTag corresponds to scala.reflect.ClassManifest and scala.reflect.api.TypeTags#TypeTag mostly corresponds to scala.reflect.Manifest, other pre-2.10 Manifest types do not have a direct correspondence with a 2.10 "Tag" type.

scala.reflect.OptManifest is not supported.
This is because Tags can reify arbitrary types, so they are always available. -
There is no equivalent for scala.reflect.AnyValManifest.
Instead, one can compare their Tag with one of the base Tags (defined in the corresponding companion objects) in order to find out whether or not it represents a primitive value class. Additionally, it's possible to simply use <tag>.tpe.typeSymbol.isPrimitiveValueClass.
There are no replacement for factory methods defined in the Manifest companion objects.
Instead, one could generate corresponding types using the reflection APIs provided by Java (for classes) and Scala (for types).
Certain manifest operations(i.e., <:<, >:> and typeArguments) are not supported.
Instead, one could use the reflection APIs provided by Java (for classes) and Scala (for types).

In Scala 2.10, scala.reflect.ClassManifests are deprecated, and it is planned to deprecate scala.reflect.Manifest in favor of TypeTags and ClassTags in an upcoming point release. Thus, it is advisable to migrate any Manifest-based APIs to use Tags.

For more information about TypeTags, see the Reflection Guide: TypeTags

Self Type: Universe
Source: TypeTags.scala
See also: scala.reflect.ClassTag, scala.reflect.api.TypeTags#TypeTag, scala.reflect.api.TypeTags#WeakTypeTag

Linear Supertypes

AnyRef, Any

Known Subclasses

JavaUniverse, Universe, Universe

Type Hierarchy

trait TypeTag[T] extends Universe.WeakTypeTag[T] with Equals with Serializable

A TypeTag is a scala.reflect.api.TypeTags#WeakTypeTag with the additional static guarantee that all type references are concrete, i.e.
A TypeTag is a scala.reflect.api.TypeTags#WeakTypeTag with the additional static guarantee that all type references are concrete, i.e. it does not contain any references to unresolved type parameters or abstract types.

Annotations
@implicitNotFound( "No TypeTag available for ${T}" )
See also
scala.reflect.api.TypeTags
trait WeakTypeTag[T] extends Equals with Serializable

If an implicit value of type WeakTypeTag[T] is required, the compiler will create one, and the reflective representation of T can be accessed via the tpe field.
If an implicit value of type WeakTypeTag[T] is required, the compiler will create one, and the reflective representation of T can be accessed via the tpe field. Components of T can be references to type parameters or abstract types. Note that WeakTypeTag makes an effort to be as concrete as possible, i.e. if TypeTags are available for the referenced type arguments or abstract types, they are used to embed the concrete types into the WeakTypeTag. Otherwise the WeakTypeTag will contain a reference to an abstract type. This behavior can be useful, when one expects T to be perhaps be partially abstract, but requires special care to handle this case. However, if T is expected to be fully known, use scala.reflect.api.TypeTags#TypeTag instead, which statically guarantees this property.
For more information about TypeTags, see the Reflection Guide: TypeTags

Annotations
@implicitNotFound( "No WeakTypeTag available for ${T}" )
See also
scala.reflect.api.TypeTags

abstract def symbolOf[T](implicit arg0: Universe.WeakTypeTag[T]): Universe.TypeSymbol

Type symbol of x as derived from a type tag.
object TypeTag extends Serializable

Type tags corresponding to primitive types and constructor/extractor for WeakTypeTags.
object WeakTypeTag extends Serializable

Type tags corresponding to primitive types and constructor/extractor for WeakTypeTags.
def typeOf[T](implicit ttag: Universe.TypeTag[T]): Universe.Type

Shortcut for implicitly[TypeTag[T]].tpe
def typeTag[T](implicit ttag: Universe.TypeTag[T]): Universe.TypeTag[T]

Shortcut for implicitly[TypeTag[T]]
def weakTypeOf[T](implicit attag: Universe.WeakTypeTag[T]): Universe.Type

Shortcut for implicitly[WeakTypeTag[T]].tpe
def weakTypeTag[T](implicit attag: Universe.WeakTypeTag[T]): Universe.WeakTypeTag[T]

Shortcut for implicitly[WeakTypeTag[T]]

Ungrouped

final def !=(arg0: Any): Boolean

Test two objects for inequality.
Test two objects for inequality.
returns
true if !(this == that), false otherwise.

Definition Classes
AnyRef → Any
final def ##(): Int

Equivalent to x.hashCode except for boxed numeric types and null.
Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.
returns
a hash value consistent with ==

Definition Classes
AnyRef → Any
def +(other: String): String

Implicit information
This member is added by an implicit conversion from TypeTags to any2stringadd[TypeTags] performed by method any2stringadd in scala.Predef.
Definition Classes
any2stringadd
def ->[B](y: B): (TypeTags, B)

Implicit information
This member is added by an implicit conversion from TypeTags to ArrowAssoc[TypeTags] performed by method ArrowAssoc in scala.Predef.
Definition Classes
ArrowAssoc
Annotations
@inline()
final def ==(arg0: Any): Boolean

The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).
The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).
returns
true if the receiver object is equivalent to the argument; false otherwise.

Definition Classes
AnyRef → Any
final def asInstanceOf[T0]: T0

Cast the receiver object to be of type T0.
Cast the receiver object to be of type T0.
Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expression List(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.
returns
the receiver object.

Definition Classes
Any
Exceptions thrown
ClassCastException if the receiver object is not an instance of the erasure of type T0.
def clone(): AnyRef

Create a copy of the receiver object.
Create a copy of the receiver object.
The default implementation of the clone method is platform dependent.
returns
a copy of the receiver object.

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
@throws( ... )
Note
not specified by SLS as a member of AnyRef
def ensuring(cond: (TypeTags) ⇒ Boolean, msg: ⇒ Any): TypeTags

Implicit information
This member is added by an implicit conversion from TypeTags to Ensuring[TypeTags] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
def ensuring(cond: (TypeTags) ⇒ Boolean): TypeTags

Implicit information
This member is added by an implicit conversion from TypeTags to Ensuring[TypeTags] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
def ensuring(cond: Boolean, msg: ⇒ Any): TypeTags

Implicit information
This member is added by an implicit conversion from TypeTags to Ensuring[TypeTags] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
def ensuring(cond: Boolean): TypeTags

Implicit information
This member is added by an implicit conversion from TypeTags to Ensuring[TypeTags] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
final def eq(arg0: AnyRef): Boolean

Tests whether the argument (that) is a reference to the receiver object (this).
Tests whether the argument (that) is a reference to the receiver object (this).
The eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:
- It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
- For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
- null.eq(null) returns true.
When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).
returns
true if the argument is a reference to the receiver object; false otherwise.

Definition Classes
AnyRef
def equals(arg0: Any): Boolean

The equality method for reference types.
The equality method for reference types. Default implementation delegates to eq.
See also equals in scala.Any.
returns
true if the receiver object is equivalent to the argument; false otherwise.

Definition Classes
AnyRef → Any
def finalize(): Unit

Called by the garbage collector on the receiver object when there are no more references to the object.
Called by the garbage collector on the receiver object when there are no more references to the object.
The details of when and if the finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
@throws( classOf[java.lang.Throwable] )
Note
not specified by SLS as a member of AnyRef
def formatted(fmtstr: String): String

Implicit information
This member is added by an implicit conversion from TypeTags to StringFormat[TypeTags] performed by method StringFormat in scala.Predef.
Definition Classes
StringFormat
Annotations
@inline()
final def getClass(): Class[_]

A representation that corresponds to the dynamic class of the receiver object.
A representation that corresponds to the dynamic class of the receiver object.
The nature of the representation is platform dependent.
returns
a representation that corresponds to the dynamic class of the receiver object.

Definition Classes
AnyRef → Any
Note
not specified by SLS as a member of AnyRef
def hashCode(): Int

The hashCode method for reference types.
The hashCode method for reference types. See hashCode in scala.Any.
returns
the hash code value for this object.

Definition Classes
AnyRef → Any
final def isInstanceOf[T0]: Boolean

Test whether the dynamic type of the receiver object is T0.
Test whether the dynamic type of the receiver object is T0.
Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression 1.isInstanceOf[String] will return false, while the expression List(1).isInstanceOf[List[String]] will return true. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.
returns
true if the receiver object is an instance of erasure of type T0; false otherwise.

Definition Classes
Any
final def ne(arg0: AnyRef): Boolean

Equivalent to !(this eq that).
Equivalent to !(this eq that).
returns
true if the argument is not a reference to the receiver object; false otherwise.

Definition Classes
AnyRef
final def notify(): Unit

Wakes up a single thread that is waiting on the receiver object's monitor.
Wakes up a single thread that is waiting on the receiver object's monitor.

Definition Classes
AnyRef
Note
not specified by SLS as a member of AnyRef
final def notifyAll(): Unit

Wakes up all threads that are waiting on the receiver object's monitor.
Wakes up all threads that are waiting on the receiver object's monitor.

Definition Classes
AnyRef
Note
not specified by SLS as a member of AnyRef
final def synchronized[T0](arg0: ⇒ T0): T0

Definition Classes
AnyRef
def toString(): String

Creates a String representation of this object.
Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.
returns
a String representation of the object.

Definition Classes
AnyRef → Any
final def wait(): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )
final def wait(arg0: Long, arg1: Int): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )
final def wait(arg0: Long): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )
def →[B](y: B): (TypeTags, B)

Implicit information
This member is added by an implicit conversion from TypeTags to ArrowAssoc[TypeTags] performed by method ArrowAssoc in scala.Predef.
Definition Classes
ArrowAssoc

TypeTags

Related Doc: package api

trait TypeTags extends AnyRef

#1 Via the methods typeTag, classTag, or weakTypeTag

#2 Using an implicit parameter of type TypeTag[T], ClassTag[T], or WeakTypeTag[T]

#3 Context bound of a type parameter

WeakTypeTags

TypeTags and Manifests

Type Members

trait TypeTag[T] extends Universe.WeakTypeTag[T] with Equals with Serializable

trait WeakTypeTag[T] extends Equals with Serializable

Abstract Value Members

abstract def symbolOf[T](implicit arg0: Universe.WeakTypeTag[T]): Universe.TypeSymbol

Concrete Value Members

final def !=(arg0: Any): Boolean

final def ##(): Int

def +(other: String): String

def ->[B](y: B): (TypeTags, B)

final def ==(arg0: Any): Boolean

object TypeTag extends Serializable

object WeakTypeTag extends Serializable

final def asInstanceOf[T0]: T0

def clone(): AnyRef

def ensuring(cond: (TypeTags) ⇒ Boolean, msg: ⇒ Any): TypeTags

def ensuring(cond: (TypeTags) ⇒ Boolean): TypeTags

def ensuring(cond: Boolean, msg: ⇒ Any): TypeTags

def ensuring(cond: Boolean): TypeTags

final def eq(arg0: AnyRef): Boolean

def equals(arg0: Any): Boolean

def finalize(): Unit

def formatted(fmtstr: String): String

final def getClass(): Class[_]

def hashCode(): Int

final def isInstanceOf[T0]: Boolean

final def ne(arg0: AnyRef): Boolean

final def notify(): Unit

final def notifyAll(): Unit

final def synchronized[T0](arg0: ⇒ T0): T0

def toString(): String

def typeOf[T](implicit ttag: Universe.TypeTag[T]): Universe.Type

def typeTag[T](implicit ttag: Universe.TypeTag[T]): Universe.TypeTag[T]

final def wait(): Unit

final def wait(arg0: Long, arg1: Int): Unit

final def wait(arg0: Long): Unit

def weakTypeOf[T](implicit attag: Universe.WeakTypeTag[T]): Universe.Type

def weakTypeTag[T](implicit attag: Universe.WeakTypeTag[T]): Universe.WeakTypeTag[T]

def →[B](y: B): (TypeTags, B)

Inherited from AnyRef

Inherited from Any

Inherited by implicit conversion any2stringadd from TypeTags to any2stringadd[TypeTags]

Inherited by implicit conversion StringFormat from TypeTags to StringFormat[TypeTags]

Inherited by implicit conversion Ensuring from TypeTags to Ensuring[TypeTags]

Inherited by implicit conversion ArrowAssoc from TypeTags to ArrowAssoc[TypeTags]

TypeTags

trait TypeTag[T] extends Universe.WeakTypeTag[T] with Equals with Serializable

trait WeakTypeTag[T] extends Equals with Serializable

abstract def symbolOf[T](implicit arg0: Universe.WeakTypeTag[T]): Universe.TypeSymbol

object TypeTag extends Serializable

object WeakTypeTag extends Serializable

def typeOf[T](implicit ttag: Universe.TypeTag[T]): Universe.Type

def typeTag[T](implicit ttag: Universe.TypeTag[T]): Universe.TypeTag[T]

def weakTypeOf[T](implicit attag: Universe.WeakTypeTag[T]): Universe.Type

def weakTypeTag[T](implicit attag: Universe.WeakTypeTag[T]): Universe.WeakTypeTag[T]

Ungrouped

final def !=(arg0: Any): Boolean

final def ##(): Int

def +(other: String): String

def ->[B](y: B): (TypeTags, B)

final def ==(arg0: Any): Boolean

final def asInstanceOf[T0]: T0

def clone(): AnyRef

def ensuring(cond: (TypeTags) ⇒ Boolean, msg: ⇒ Any): TypeTags

def ensuring(cond: (TypeTags) ⇒ Boolean): TypeTags

def ensuring(cond: Boolean, msg: ⇒ Any): TypeTags

def ensuring(cond: Boolean): TypeTags

final def eq(arg0: AnyRef): Boolean

def equals(arg0: Any): Boolean

def finalize(): Unit

def formatted(fmtstr: String): String

final def getClass(): Class[_]

#2 Using an implicit parameter of type `TypeTag[T]`, `ClassTag[T]`, or `WeakTypeTag[T]`

`WeakTypeTag`s